FIFTH REPORT OF THE DIRECTOR 1908 



13 



extensive fault and thrust movements we may infer the existence of 

 similar ones to the south and east which may serve to explain 

 some of the difficult phenomena there exhibited, in the presence, 

 or absence, or metamorphism of the younger rocks and in the 

 intrarelationships among the basal gneisses. 



Thrust movements of considerable magnitude, measured by 

 hundreds or a few thousand feet, are occasional and numerous 

 smaller adjustment faults are frequent. No evidence has been 

 secured of displacements on the west of the Highlands axis which 

 are supposed to have carried the strata for distances of miles ; and 

 were the strata that once overlay this region to be restored it is 

 probable that the extent of overturning and overthrusting could be 

 measured by a few thousand feet. 



The present altitude of the Highlands with respect to the 

 neighboring Paleozoics serves in general as no indication of the 

 former early relations between these great rock divisions. Folding, 

 faulting and erosion, each or all, perhaps, repeated, have combined 

 to efface the original early relationship. We seize upon what 

 time has left with the hope of untangling the maze that now con- 

 fronts us. 



The sky-line of the summits of the Highlands knobs and ridges 

 even with that of more distant hills among the younger rocks 

 - presents the aspect of a former base level that is unmistakable. 

 What masses of overlying strata have been removed from these 

 crystalline Highlands rocks we can only guess from the thickness 

 of those to the northward, the character of the folding there, and 

 the length of time the region has suffered denudation. Lofty 

 mountain ridges were reduced from alpine bights to a peneplain. 

 From their stumps we are obliged to construct our imperfect history 

 of the region. 



Perhaps in no other place in eastern North America is the con- 

 tact of the lowest Paleozoics on the underlying Precambric better 

 preserved than on the flanks of the Highlands of southeastern New 

 York and nowhere is it better shown than on the northern slope 

 of the FishkilL mountains. 



Here for considerable distances the basal Cambric quartzite rests 

 unconformably upon the folded basal gneisses. The forces of 

 erosion that have removed the quartzite and the overlying lime- 

 stones, which once filled the northern valleys of these mountains, 

 have cut deeply enough to expose a faulted block of the basal 

 Paleozoics where the relationships have been well preserved. The 

 usual abnormal relationships present along this border as a result 



